adjust text size:

Foreign Service

A charging system problem can complicate a diagnosis and fool you when you least expect it. The key is to keep your wits about you and test the charging system sooner in your diagnosis rather than later.

Last month’s column described a bad alternator ground that could cause a nasty overcharge condition on popular Honda Civics. No sooner had I finished that column than I heard about another newsworthy Honda charging system problem—this time on an Accord. Here’s what happened:

Justin Schlossberg, a technician at Triad Auto, a Honda specialty shop in Greensboro, NC, was diagnosing a 1998 Accord equipped with the 2.3L engine. The car arrived with an illuminated MIL and DTC P1381 stored in computer memory. This trouble code indicates an intermittent CYP sensor signal. The CYP (cylinder position) sensor is inside the distributor. The guys at another shop had already replaced the distributor assembly, but the trouble code returned.

When Schlossberg tried to get more vehicle history, he learned that the original symptoms were the illuminated Check Engine light and generally poor performance. After checking some basics, he cleared the CYP trouble code and road-tested the car. At first, the engine restarted fine but initially idled very rough. Soon the MIL came on and the engine idled somewhat smoother. Sure enough, the computer had stored the same code again. During the road test, the Accord accelerated sluggishly and roughly. The misfiring wasn’t severe and no misfire codes were stored, but the engine wasn’t running smoothly, either.

Schlossberg decided to go right to the CYP sensor. The screen capture in Fig. 1 below shows the sensor’s signal at idle. On the one hand, this looks very much like a basic AC voltage pattern. On the other hand, the horizontal part of the signal has a telltale, hashy-trashy look about it.

On this Accord’s charging system, terminal IG is the excitation (turn-on command) terminal for the charging system. Schlossberg knew that terminal IG is in the alternator’s square, green harness connector. Therefore, removing this connector would shut off the alternator.

The screen capture in Fig. 2 on page 14 shows the results of disconnecting the alternator connector at idle. Granted, this pattern is on a 10-volt scale instead of a 5-volt scale. But the severe hash evident in Fig. 1 disappeared when Schlossberg disconnected the alternator. Likewise, this hash on the pattern returned as soon as he reconnected the alternator connector.

You have to replace the entire distributor assembly in order to replace the CYP sensor. Schlossberg figured all along that the chances of getting a defective new CYP sensor were slim to none. But now a simple scope test revealed another culprit—electrical interference from a failing alternator. The screen capture in Fig. 3 above shows the results of a scope test of the Accord’s alternator. Obviously, this is anything but the clean, consistent ripple pattern we expect from a good alternator. No, this thing had an open diode. Replacing the alternator fixed this Accord.

Note that this Accord’s charge indicator light never turned on when the car was at the other shop or when Schlossberg had it. What’s more, the owner never mentioned any electrical symptoms; the illuminated Check Engine light was the reason the car came into the shop. (Sure, there’s always the possibility that he was so focused on the Check Engine light that he forgot about other symptoms that were present.)

We’ve seen this lots of times: An alternator with one open diode may very well produce enough voltage during no-load or light-load conditions to turn out a charge indicator light. Therefore, it’s not hard to understand why obvious undercharge symptoms aren’t present.

One solution is to routinely perform a charging system test before you get too involved in a diagnosis. At some shops, a routine charging system test is part of the shop’s basic diagnosis. Maybe it should be in yours as well. After all, verifying proper operation of the charging system allows you to eliminate it as a cause of other, seemingly unrelated problems.